The invention relates to a low-pressure mercury vapour discharge lamp having a discharge vessel filled with mercury and a rare gas, which discharge vessel comprises two substantially parallel, mainly rectangular flat glass plates which are located at a relatively short distance from each other and which are transparent to light, said plates being connected together in a gas-tight manner proximate their circumference and forming upright walls. Electrodes are arranged on the outer side of the discharge vessel between which a discharge is maintained in the discharge vessel during operation. A lamp of this type is known from Japanese Patent Application 61-4152(A) laid open to public inspection.
These flat lamps are used as light sources for rear exposure of information display devices comprising liquid crystal material and having a flat display screen.
Such a display device comprises, for example a chamber having two electrically insulating transparent plates between which liquid crystal material is present. The surfaces of the insulating plates facing the liquid crystal material are provided with planar electrodes. Dependent on the potential difference between the electrodes and a special polarizer, bright symbols are displayed against a dark background, and conversely, by means of the light source placed behind the display device. The said light source exposing the display device particularly on the rear side is preferably a fluorescent low-pressure mercury vapour discharge lamp having a flat rectangular discharge vessel as described inter alia in the said Japanese Patent Application. Such a light source has a high brightness and the rear side of the display device is exposed over a maximum possible surface area.
In the flat discharge vessel of the known lamp the electrodes are arranged on the outer side of the flat substantially rectangular glass plates and they are formed as thin transparent conducting layers (comprising, for example conducting indium oxide) which layers extend substantially throughout the surface. The glass plates are substantially rectangular but this is also understood to mean a square, though small deviations such as roundings may be possible. During operation of the lamp a high-frequency alternating voltage is maintained between the electrodes causing ultraviolet radiation in the discharge vessel which is converted into visible light by a luminescent layer provided on the inner wall of the discharge vessel. However, it has been found that due to the relatively large surface area of the electrodes in the known lamp the discharge readily contracts in the discharge vessel during operation, resulting in local intensity differences. To prevent such a contraction of the discharge, the distance between the said plates should be determined very accurately. Such an accuracy requirement must also be imposed on the thickness of the said glass plates. This is troublesome and hence timeconsuming and costly, notably in lamps manufactured in a mass production process.